The present exemplary embodiments relate to a method for layering phosphors in a light emitting diode (“LED”) device to produce efficient lighting devices and minimize non-linear effects. It finds particular application in conjunction with LEDs, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Light emitting diodes (“LEDs”) are well-known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. However, the potential for general illumination is also enormous. Originally, the most efficient LEDs emitted light having a peak wavelength in the red region of the light spectrum, i.e., red light. However, a type of LED based on a gallium nitride (GaN) die has also been developed that can efficiently emit light having a peak wavelength in the blue and/or UV region of the spectrum. This type of LED can now provide significantly brighter output light.
Since blue and UV light have a shorter peak wavelength than red light, the blue/UV light generated by a GaN-based LED die can be readily converted to light having a longer peak wavelength. It is well known in the art that light having a first peak wavelength (the “primary light”) can be converted into light having a longer peak wavelength (the “secondary light”) using a process known as phosphorescence. The phosphorescence process involves the absorption of the primary light by a photoluminescent phosphor material, which acts to excite the atoms of the phosphor material to emit the secondary light. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength.
According to U.S. Pat. No. 5,998,925, a phosphor comprised of (Y0.4Gd0.6)3Al5O12:Ce, for example, is used to convert a portion of a blue emitted light from a GaN die to yellow light. The total emission of the LED device, consisting of blue GaN device generated light and yellow phosphor emitted light is a generally white light. According to U.S. Pat. No. 6,522,065, a similar result can be achieved using a UV emitting GaN die. In this patent a phosphor comprised of Ca1.94Na1.03Eu0.03Mg2V3O12, for example, is used to convert at least a substantial portion of the GaN emitted UV light to white light.
There are two primary problems in the white LED lamps described above, which are particularly present in UV-LED devices. First, if the absorption of the phosphor at the emitted UV wavelength is not high, then there is a significant amount of radiation emitted from the die that bleeds through the phosphor coating. Since the eye does not perceive UV radiation well, the overall lumen output of the lamp is reduced. Similarly, too much blue light bleed-through disturbs the lamp color. To combat this bleedthrough and reduce the required thickness of the phosphor layer, new phosphors have been developed with increased quantum efficiency (QE) and greater absorption of LED radiation. However, this increased QE and absorption only contributed to the second problem, wherein phosphors with slow decay times that are subjected to high incident fluxes from the LED chip can lead to saturation. This loss mechanism can severely limit the efficiency of white light or saturated color light sources. In addition, other requirements, such as a substantially constant color point with a varying LED drive current, are virtually impossible to meet if there is significant phosphor saturation.
Thus, a need exists for a method to select and/or arrange phosphors in an LED lamp to minimize saturation of the phosphors and thereby improve lamp efficiency and maintain a stable lamp color through a range of drive currents.